1. Field
The field relates to an organic light emitting display device, and more specifically to an organic light emitting display device capable of displaying an image having a uniform luminance and simultaneously minimizing the number of transistors.
2. The Related Technology
In recent years, there have been may attempts to develop various flat panel displays with less weight and volume than a cathode ray tube, which are problematic in the cathode ray tube. The flat panel display includes a liquid crystal display, a field emission display, a plasma display panel, an organic light emitting display device, etc.
Amongst flat panel display devices, the organic light emitting display device displays an image using an organic light emitting diode which generates the lights by means of recombination of electrons and holes. Such an organic light emitting display device has an advantage that it has a rapid response time and may be also driven with low power consumption.
FIG. 1 is a circuit view showing a pixel of a conventional organic light emitting display device.
Referring to FIG. 1, the pixel 4 of the conventional organic light emitting display device includes an organic light emitting diode (OLED), and a pixel circuit 2 connected to a data line (Dm) and a scan line (Sn) to control the organic light emitting diode (OLED).
An anode electrode of the organic light emitting diode (OLED) is connected to the pixel circuit 2, and a cathode electrode is connected to a second power source (ELVSS). Such an organic light emitting diode (OLED) generates the light having a luminance corresponding to an electric current supplied to the pixel circuit 2.
The pixel circuit 2 controls a current supplied to the organic light emitting diode (OLED) according to a data signal supplied to the data line (Dm) while a scan signal is supplied to the scan line (Sn). For this purpose, the pixel circuit 2 includes a second transistor (M2) connected between a first power source (ELVDD) and the organic light emitting diode (OLED); a first transistor (M1) connected to the second transistor (M2), the data line (Dm) and the scan line (Sn); and a storage capacitor (Cst) connected between a gate electrode and a first electrode of the second transistor (M2).
The gate electrode of the first transistor (M1) is connected to the scan line (Sn), and the first electrode is connected to the data line (Dm). And, the second electrode of the first transistor (M1) is connected to one terminal of the storage capacitor (Cst). The first electrode is either a source electrode or a drain electrode, and the second electrode is the electrode different than the first electrode. For example, a second electrode may be a drain electrode if the first electrode is a source electrode. The first transistor (M1) is connected to the scan line (Sn) and the data line (Dm) is turned on while a scan signal is supplied to the scan line (Sn), thereby supplying a data signal, from the data line (Dm), to the storage capacitor (Cst). As a result, the storage capacitor (Cst) stores a voltage corresponding to the data signal.
The gate electrode of the second transistor (M2) is connected to one terminal of the storage capacitor (Cst), and the first electrode is connected to the other terminal of the storage capacitor (Cst) and to the first power source (ELVDD). The second electrode of the second transistor (M2) is connected to the anode electrode of the organic light emitting diode (OLED). The second transistor (M2) controls current according to a voltage value stored in the storage capacitor (Cst), wherein the current flows from the first power source (ELVDD) to the second power source (ELVSS) through the organic light emitting diode (OLED). The organic light emitting diode (OLED) generates light corresponding to the current supplied to the second transistor (M2).
However, the pixel 4 of has a problem that it is impossible to display an image having a uniform luminance. This is so because a threshold voltage of the second transistor (M2) (the drive transistor) in each of the pixels 4 has different values because of schedule variance, and other factors. Because the threshold voltage of the drive transistor has different values, light having different luminance is generated in the organic light emitting diode (OLED) due to the difference in the resulting current of the drive transistor even if a data signal corresponding to the same grey levels is supplied to a large number of pixels 4.
In order to solve the above problems, there has been proposed a configuration in which transistors are further added to each of the pixels 4 to compensate for a threshold voltage of the drive transistor. Actually, there has been proposed a configuration in which 6 transistors and one capacitor are used for each of the pixels 4 to compensate for a threshold voltage of the drive transistor. However, the size of the pixels 4 is increased if the 6 transistors are included in each of the pixels 4. Also, the possibility of erroneous operation is increased with a large number of transistors in the pixels 4, resulting in the reduced yield of the organic light emitting display device during manufacturing.